I never was a fan of those cedar blocks for grilling fish. I think this is a great idea, though! The Husband loves lemon on his fish, too - Bonus! Grill your fish on a bed of lemons to infuse flavor & prevent sticking to the grill.
Cut up a disposable water bottle and keep the neck and top, as in photo. Insert the plastic bag through the neck and screw the top to seal. The bag is made to be air-tight, such that water will not leak, the secret lies with the top and screw cap!